1. Field of the Invention
This invention relates to multi-layer transformers, more specifically, to multi-layer transformers with improved magnetic coupling and dielectric breakdown voltage between windings in the multi-layer transformers.
2. Description of Related Art
The use of multi-layer transformers is widely known. In general, a multi-layer transformer is constructed with the following process. A magnetic material, for example, ferrite, is cast into tape. The tape is then cut into sheets or layers, and vias are formed at the required locations in each of the tape layers to form conductive pathways. Conductive pastes are subsequently deposited on the surface of the tape layers to form the spiral windings which terminate at the vias. After that, a number of the tape layers with corresponding conductive windings are stacked up with vias in appropriate alignment to form a multi-turn transformer structure. The collated layers are joined together by heat and pressure. The structure is then transferred to a sintering oven to form a homogenous monolithic ferrite transformer. With the above process, many transformers can be made at the same time by forming an array of vias and conductive windings on the surface of the ferrite layers. The transformer may be singulated pre or post firing. FIGS. 1-2 show an example of a traditional ferrite transformer formed by using the above process.
However, a transformer constructed in the above process has a uniform magnetic permeability throughout the multi-layer structure. Some of the magnetic flux lines generated by the conductive windings cut through the adjacent windings. For example, in a structure where primary windings and secondary windings are disposed in an interleaving relationship on different layers, not all flux lines generated by the primary windings cut through the secondary winding. This yields inefficient flux linkage between the primary windings and the secondary windings. The efficiency of the flux linkage between primary windings and secondary windings can be determined by a magnetic coupling factor. Generally, the magnetic coupling factor between primary and secondary windings is defined as .alpha.= ##EQU1##
wherein L.sub.pri represents primary magnetizing inductance, and L.sub.leak represents the inductance measured across the primary winding with the secondary winding shorted. It has been determined empirically that coupling is a function of proximity between windings. A transformer (as shown in FIGS. 1 and 2) with a uniform permeability has a magnetic coupling factor of 0.83.
Though a closer spacing between the windings in adjacent layers can obtain a higher magnetic coupling factor, the ferrite layers must be made thick enough to withstand a minimum voltage where no dielectric breakdown occurs between the windings. For example, the thickness of a typical NiZn ferrite material requires more than 7 mils to withstand 2400 VAC.
In order to obtain a high magnetic coupling factor, another method has been suggested in U.S. Pat. No. 5,349,743. The '743 patent suggests forming apertures and sing two separate materials to limit the magnetic flux paths to a well defined core area to increase coupling. However, this method is very expensive and limits transformer miniaturization due to the need to make apertures and fill them with a different material than the tape.
Thus, there is a need in the art for an improved multi-layer transformer with a higher magnetic coupling between the windings. Also, there is a need for such an improved multi-layer transformer to be constructed in a lower cost and smaller size, and/or to be readily mass producable in an automated fashion, as well as to meet regulatory safety requirements.